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Combination pills usually work by preventing a person's ovaries from releasing eggs (ovulation). They also thicken the cervical mucus, which keeps sperm from joining with an egg. The hormones in combination and progestogen-only pills also thin the lining of the uterus. This could prevent pregnancy by interfering with implantation of a blastocyst.

All contain an estrogen, ethinyl estradiol or mestranol,[1][2] in varying amounts, and one of a number of different progestogens. (Regarding the estrogen, the inactive 3-methyl ether of ethinyl estradiol, which must be metabolized by the liver into the active ethinyl estradiol; 50 µg of mestranol is equivalent to only 35 µg of ethinyl estradiol and should not be used when high-dose [50 µg ethinyl estradiol] estrogen pills are needed; mestranol was the estrogen used in the first oral contraceptive, Enovid). They are usually taken for 21 days with then a seven-day gap during which a withdrawal bleed (often, but incorrectly, referred to as a menstrual period) occurs. These differ in the amount of estrogen given, and whether they are monophasic (the same dose of estrogen and progestogen during each of the 21 days) or multiphasic (varying doses). The introduction of extended-cycle monophasic pills (i.e. Seasonale) has shown that the withdrawal bleeding intervals can be decreased.

These are typically given as 21 tablets of estrogen and progestogen, followed by seven tablets of placebo or an iron supplement,[3][4] although some newer formulations contain more active tablets and fewer placebos. Everyday regimens (Microgynon 30 ED, Femodene ED, Logynon ED), which include seven inactive placebo pills, are rarely used in UK practice.[5] Different formulations contain different amounts of estrogen and progestogen:

Progestogen-only pills (POPs) use progestogen alone with doses taken continuously and no gap between packs taken. People who use them may experience irregular light bleeds, and whilst irregular in the first few months of taking, usually settles to a regular pattern in time.

Generally oral contraceptives should not be used in women who currently have the following conditions:

Thrombophlebitis or thromboembolic disorders

A past history of deep vein thrombophlebitis or thromboembolic disorders

Cerebrovascular or coronary artery disease (current or history)

Valvular heart disease with thrombogenic complications

Severe hypertension

Diabetes with vascular involvement

Headaches with focal neurological symptoms

Major surgery with prolonged immobilization

Known or suspected carcinoma of the breast or personal history of breast cancer

Carcinoma of the endometrium or other known or suspected estrogen-dependent neoplasia

Undiagnosed abnormal genital bleeding

Cholestatic jaundice of pregnancy or jaundice with prior pill use

Hepatic adenomas or carcinomas, or active liver disease

Known or suspected pregnancy

Hypersensitivity to any component of the product

More comprehensive guidelines that include analysis of risks and benefits can be found in the World Health Organization Medical Eligibility for Contraceptive Use Guidelines[11] which are reflected in the CDC Medical Eligibility for Contraceptive Use Guidelines.[12]